I dont think you understand what a railgun is.
\"slow\" is by definition the first thing that disqualifies something from being a railgun. \"a rail gun can accelerate a projectile up to 52,493 feet (16,000 meters) per second. And while current Navy guns have a maximum range of 12 miles, rail guns can hit a target 250 miles away in six minutes.\"
This means a slow projectile that has pitiful range is out of the question. The problem is for a railgun to be implimented it would need a base size, or a set minimum dimension to work. I would say 1x10x10 (tallxwidexlong), because the range would be so high and the velocity would be so high 1 block should have that kind of capabilities. It should eat a lot of power, and regardless of size should have a minimum of 20 seconds for reload. The last thing we need is Railgun bullets flying around every 2 seconds.
Furthermore, i would say a railgun would need to eat up quite a bit of power per shot, rising statically, not exponetially so its feasable to make large RG\'s. I would say that the size should scale exponentially, probably like power tanks, but to a lesser degree so that the more you have the more powerful it gets, but should require atleast 1,000 blocks before it starts to really increase drastically. Damage i would have to say would need to be higher then AMC guns, with less AoE then a rocket of any type or size but more speed than any of them combined, it is a railgun afterall. I would also say the guns parts should cost more then any of the other cannons, simply because it will be so powerful on large ships. I would also say that if the gun was to fire on shields it should do about 2/3 the damage, because they can be quite powerful.
Generally lacking of details OP. If you want something added you really need to think it through.
http://science.howstuffworks.com/rail-gun.htm
Also:
http://www.youtube.com/watch?v=y54aLcC3G74
You can see the loading is very slow, and teh size is pretty big.